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CHAPTER
GEOLOGICAL HAZARDS
9
“If you thought that science was certain well, that is just an error on your part.”
Richard Feynman
CHAPTER OUTLINE
9.1Introduction����������������������������������������������������������������������������������������������������������������������������������� 287
9.2Physical Properties������������������������������������������������������������������������������������������������������������������������ 290
9.3Decomposition or Dissociation�������������������������������������������������������������������������������������������������������� 293
9.4Hydrate Impact on Climate�������������������������������������������������������������������������������������������������������������� 295
9.5Geological Hazards������������������������������������������������������������������������������������������������������������������������ 300
9.5.1Storrega Slide-Norwegian Sea.................................................................................... 307
9.5.2Cape Fear Slide − Atlantic Margin.............................................................................. 311
9.5.3Beaufort Sea............................................................................................................ 314
9.5.4Cascadian Margin..................................................................................................... 316
9.5.5Hydrate-Associated Risks for Oil and Gas Exploitation.................................................. 316
References������������������������������������������������������������������������������������������������������������������������������������������� 318
9.1 INTRODUCTION
Gas hydrates occur in different geological and tectonic environments and their stability is delicately
balanced under suitable thermobaric conditions. Gas hydrates as a potential unconventional resource
may provide a viable alternative for future energy requirements. It is estimated that the shallow subsurface gas hydrate reserve on the continental margins and in permafrost regions combined holds twice the
amount of carbon of all other hydrocarbon resources put together (Sloan, 2006). Enormous amounts of
methane are believed to be trapped by hydrates, both in the hydrate crystal structure itself as well as free
gas in sediments beneath the hydrate deposits. Primarily, gas hydrates occupy the pore spaces in the
sediments and cement them together. They are also formed in secondary voids in fractures and joints.
During growth, hydrates create their own space by disturbing the sediment matrix (Suess et al., 1999a).
Pore-filling morphologies of gas hydrates replace pore fluid between grains of sediment; this gas
hydrate may or may not cement grains together. Grain-displacing gas hydrate does not occupy the pore
volume between grains; instead it forces the grains apart, forming veins, layers, and lenses of pure gas
hydrate. Grain-displacing hydrates may cover a vast range of sizes, from thin veins possibly only a few
microns thick to nodules of tens of centimeters or even meters in diameter (Fig. 9.1).
Grain-displacing and pore-filling are not equivalent to the terms “massive” and “disseminated” but
these terms apply to cores which have already undergone gas hydrate dissociation, where massive gas
Geological Controls for Gas Hydrates and Unconventionals. http://dx.doi.org/10.1016/B978-0-12-802020-3.00009-6
Copyright © 2016 Elsevier Inc. All rights reserved.
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FIGURE 9.1
Microstructural models of hydrate-bearing sediments. In the first five of the six models, gas hydrates (blue)
are evenly distributed throughout the sedimentary grains (tan) to a first approximation. Hydrate may occur as
cement at grain contacts (top left), as coating on grains (top right), as a component of the grain matrix (middle
left), or as pore-filling material (middle right). The fifth model considers sedimentary grains as inclusions in
a hydrate matrix (bottom left). The sixth model (bottom right) depicts hydrates as nodules or fracture-fill on
fine-grained, low-permeability sediments. These models are used to simulate the response of hydrate-bearing
sediments to logging and seismic measurements. (Source Dai et al., 2008.)
hydrate is still visible, and disseminated gas hydrate is invisible to the naked eye, and may have already
been completely dissociated (Holland et al., 2008). Disseminated hydrates have a tendency to occupy
coarse-grained clastic sediments with higher porosity and permeability and contain fine-grained sediments
in remotely connected geological structures. In subsurface earth they may exist as individual grains or
particles disseminatedthrough the sediments but also may cement the sediment grains as nodules and laminate, layers of different shapes and sizes with dimensions of the order of a few centimeters or less (Collett,
2003). Massive gas hydrate samples have been recovered during drilling in the Malik gas hydrate research
well in the Mackenzie Delta formed in high porosity coarse-grained and gravel rocks (Dallimore et al.,
1999). Drilling over the Hydrate Ridge in the Cascadian margin has shown that gas hydrates are found to
occur in fine-grained sediments confined to the fractures and joints mostly running parallel to the bedding
planes and in some places in the fractures oblique to bedding planes (Suess et al., 1999b). Gas hydrate
deposits of different shapes and sizes, such as veins and layers of comparable size and those with spherical
to oblate features, sometimes with edges (considered as nodules) and massive lens-shaped hydrate accumulations, were obtained during drilling over the Blake Ridge (Site 570: Shipboard Scientific Party, 1985).
9.1 Introduction
289
Low molecular weight gases, such as hydrogen and helium, which are smaller than the small cavities (3.8 Å), and high molecular weight gases, such as pentane, hexane, and paraffin hydrocarbons,
which are bigger than the large cavities (>9 Å), are unsuitable for the formation of hydrates under natural conditions. However, in recent times attempts have been made to synthesize hydrates with low
molecular weight gases in the laboratory. Most low molecular weight hydrocarbon and nonhydrocarbon gases such as oxygen, hydrogen, nitrogen, carbon dioxide, methane, hydrogen sulfide, argon, and
krypton as well as some higher hydrocarbons and freons are trapped in the cavities and form hydrates
at suitable temperatures and pressures. Without the support of the trapped molecules, the lattice structure of hydrate clathrates would collapse into a conventional ice crystal structure or liquid water. When
the cavities are fully occupied by guest molecules of single chemical components then the composition
can be expressed in terms of a definite formula. However, in nature, depending upon the physical conditions (pressure, temperature) and the availability of suitable gases, only parts of the voids are filled with
guest molecules (degree of saturation).
Earlier studies had suggested that methane, the main gaseous component of gas hydrate, is generated
locally due to the decomposition of organic content and reduction of carbon dioxide in sediments. Methane is also produced due to the thermochemical, i.e., thermal degradation and cracking, process at elevated temperatures (50°C to 150°C) and extended periods of heating (Rice, 1993). Fracturing and other
weak zones in the subsurface earth provide conduits to bring thermogenic methane to shallower levels,
where the conditions are suitable for gas hydrate formation. Methane gas hydrate was assumed to be
uniformly distributed wherever pressure and temperature conditions were appropriate and generated
methane was present in large quantities (Trofimuk et al., 1973). The higher quantum of methane required
for formation of gas hydrates has led to the conclusion that methane has to be remotely brought into the
hydrate stability zone to enhance methane concentration, which is a prerequisite for the formation of gas
hydrates (Hyndman and Davis, 1992). Subsequent studies have revealed a heterogeneous global distribution of gas hydrates and this may be due to the advective transport in supplying sufficient methane-rich
fluids for hydrate formation (Buffett and Archer, 2004; Wood and Jung, 2008). Geological weak zones
such as faulting or fracturing provide conduits for the migration or seepage of methane-rich fluids in the
gas hydrate stability region. Thus it is expected that hydrates not only exist under suitable temperature
conditions but are also likely to get confined to weak geological (faulting/fracture/joints) environments.
The estimates of gas hydrate as a resource potential have been quite speculative and uncertain ever
since first attempts were made to quantify them in the 1970s (Kvenvolden, 1999). The resource estimates were primarily based on the identification of the bottom-simulating reflector (BSR) in the multichannel seismic reflection data and the observation of geological proxies such as pockmarks and
venting gas through the sedimentary layers and the water column. Parameters such as the thickness of
the hydrated layer, porosities, saturation in sediment pores, areal extent, volume, and hydrate and gas
yield were considered for estimation of the global gas content in hydrates (Collett, 2003; Milkov,
2004). These estimates are being modified in view of new information about their formation processes,
development of new techniques, high-resolution data acquisition, and ground truth by well log data
acquisition or interpretation and obtaining hydrate samples under pressurized conditions. The global
estimates of methane gas in hydrate shows large variations. During the 1970s and 1980s, the gas content (mainly methane gas) was estimated to be of the order of 1017−1018 m3; this fell in the 1980s and
1990s to the order of 1016 m3. Recent estimates (Milkov, 2004) suggest that the gas hydrates reserves
may be the order of 1014−1015 m3 (Fig. 9.2, Milkov, 2004). The global estimates of methane gas contained in gas hydrates have substantially reduced from the 1970s to the present, a significant reduction
by a factor of 2−4 The estimated quantum of gas hydrates exceeding 1000 Gt (high probability) and in
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FIGURE 9.2
Global estimates of the volume of hydrate-bound gas in marine sediments versus the year in which the estimate
was made. (Data from Milkov, 2004.)
the range of 1000–10,000 Gt, equivalent to ∼ 2000–20,000 trillion cubic meters (medium probability),
was calculated at a recently held workshop (Bohannon, 2008).
Suitable physical conditions, enormous primary productivity, and influx of terrestrial organic matter
make continental margins a most attractive proposition for the commercial exploitation of gas hydrates
to balance supply and demand of oil and gas. However, owing to their accumulation in deepwater,
lower saturation (17−20%), which is about 3% concentration in sediments, increases the likelihood of
geological hazards that come about as a result of their dissociation (Maslin et al., 2010). Furthermore,
limitations of the present-day technology mean that commercial exploitation of gas hydrates over the
continental margins does not look feasible in the immediate future. Gas hydrate morphology describes
the relationship between gas hydrates and the surrounding marine sediments. The morphology of gas
hydrates determines the basic physical properties of the sediment−hydrate matrix.
9.2 PHYSICAL PROPERTIES
The presence of gas hydrates in sediments dramatically alters some of the normal physical properties
of the sediments. An understanding of the physical properties of the hydrate-bearing sediments is necessary for interpretation of geophysical data collected in the field setting, including borehole and slope
stability analyses, reservoir simulation, and production models. The geomechanical properties of
hydrate-bearing sediments are not yet fully understood. Laboratory experiments, small scale physical
modeling, and theoretical analyses have been undertaken to estimate their behavior with regard to
9.2 Physical Properties
291
degree of saturation, grain size, porosity, and other parameters; such attempts in turn may provide “a
priori” knowledge to assess the impact of geological processes that govern submarine slope failure and
mitigate the risks during oil and gas exploitation.
Gas hydrates cement between sediment grains or act as cement around contacting grains, and provide grain-to-grain cohesion and frictional resistance to the shallow soft sediments, where gas hydrates
are likely to occur. The strength of methane hydrate-bearing sediments is a fundamental parameter
required for the assessment of methane production from a hydrate accumulation and assessment of
submarine slope stability and its productivity. Studies undertaken so far demonstrate that the presence
of hydrates will increase strength and stiffness and decrease permeability of sediments. In fact, laboratory experiments indicate that the strength of pure hydrates can be 20 times that of pure ice, a contrast
that increases with lower temperatures (Durham et al., 2003). However, when contained within the
sediments, laboratory results show increases in the strength of hydrate-bearing sediments over hydratefree sediments, with hydrate- and ice-bearing sediments having similar strengths (Masui et al., 2005,
2008; Edinuma et al., 2005). The strength of hydrate-bearing sediments is the function of hydrate saturation, strain rate, temperature, consolidation stress, grain size, density, and cage occupancy (Winters
et al., 2004). Even at low concentrations, intergranular hydrate may thus influence the formation
strength depending on its articulation within the sediment host. At higher concentrations, the strengthening effects from pore-filling gas hydrate can be obtained by analogy to strength enhancement in
frozen soils or from general theory and experiments with rheologies of aggregate mixtures. Hydrated
sediment specimens show increase in strength with hydrate saturation (Durham et al., 2005).
The formation of hydrates between the sand particles contributes to the increases in cohesion but has
little impact on the friction angle (Masui et al., 2005; Suzuki et al., 2008). Testing to date indicates that
stiffness, cohesion, and dilation increase with increase in saturation, while the friction angle remains
unaffected. Modeling results on synthetic hydrates (Yun et al., 2007) show that the stress−strain behavior
of hydrate-bearing sediments is a complex function of particle size, confining pressure, and hydrate concentration. The mechanical properties of hydrate-bearing sediments at low hydrate concentrations (probably <40% of pore space) appear to be determined by stress-dependent soil stiffness and strength. At high
hydrate concentrations (>50% of pore space), the behavior becomes independent of stress because the
hydrates control both the stiffness, strength, and possibly the dilative tendency of sediments by effectively increasing interparticle coordination, cementing particles together, and filling the pore space. The
contribution of cementation to the shear strength of hydrate-bearing sediments decreases with increasing
specific surface of the soil minerals. Hydrates contained in the pore spaces accentuate the pore pressure
response, shear decrease in the coarse-grained sediments, and shear increase in the fine-grained sediments. The presence of a gas phase dampened the pore pressure response (Winters et al., 2007).
The thermal properties of hydrate-bearing rocks are a controlling factor for all processes involving
the formation and decomposition of gas hydrates in nature which are inevitably coupled with the transport of heat within the formation. The stability of gas hydrates is controlled more by temperature than
pressure. In this context, detailed knowledge about the thermal properties of rocks containing gas
hydrates is required in order to quantify processes involving the formation and decomposition of gas
hydrates in nature. The measured heat flow in a region is the product of thermal conductivity and the
geothermal gradient. Thermal conductivity describes the material’s ability to transfer heat. Measurement of in situ thermal conductivity provides information about the rock matrix where hydrates are
likely to be present. The mineralogical composition of the rock matrix determines the thermal conductivity of the host rock. Geological features such as salt-bearing structures (e.g., diapirs) have good
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thermal conductivity and shale diapirs have lower thermal conductivities. For a constant heat flow, the
isotherm has to undulate by increasing or decreasing the geothermal gradients due to changes in thermal conductivities of the host rock (detailed in Chapter 4). Such variations in geothermal gradients
affect the thickness of the gas hydrate stability zone. The thermal conductivity of pure methane hydrate
was determined as 0.45 W/mK at 216.2 K (Cook and Leaist, 1983). The thermal conductivity of pure
hydrate is therefore about 20% lower than the thermal conductivity of water, and up to 80% lower than
that of ice. Owing to the low thermal conductivity of pure hydrate, the presence of gas hydrate in sediments may significantly alter the bulk rock thermal conductivity and the geothermal gradient within the
hydrate-bearing formations (Ruppel, 2000).
At low saturations (Sh ∼30−35%), thermal conductivity of investigated gas saturated sediments
varied slightly and did not exceed 1−2% when compared to the host rock. In contrast there were significant changes in thermal conductivity of gas-saturated sediments above 35−40%. Thus, in the sand
sample with the increase of (Sh) from 0 to 38% the thermal conductivity increased about 8% (from 1.85
W/mK to almost 2.0 W/mK). Unlike samples without gas hydrates, an abnormal fall in thermal conductivity was observed in hydrate-saturated sediments. Thus, the difference in the values of thermal conductivity of hydrate-containing sediments and samples without hydrates can reach 10% or even higher;
this distinction was 20% in the sand sample, and more than 100% in samples of sandy loam sand. Such
behavior of thermal conductivity of hydrate-saturated sediments is caused not only by a difference in
the values of thermal conductivity of porous ice and porous hydrates but also by structurally textural
changes in hydrate-saturated sediments during freezing. Decrease of thermal conductivity was observed
during hydrate accumulation in sediments. Thus, in the sand sample (sand% ∼22 in the sediment mix)
at Sh = 0% thermal conductivity was 2.00 W/mK, and at Sh = 38% thermal conductivity was 1.73 W/mK.
In this case, decrease of the thermal-physic parameter was 14% (Chuvilin and Buhanov, 2011).
The hydrate target occurs in the shallow section (hundreds of meters in depth), and is manifested by
subtle resistivity contrasts (a few ohm-m). Electrical sounding provides an alternate method of locating
hydrate in the absence of a BSR (Yuan and Edwards, 2000). Hydrates form from pore fluid but usually
exclude salt from the pore fluid, so they have high electric resistivity just as ice and sediments containing hydrates have a higher resistivity compared to sediments without gas hydrates (Judge, 1982). Laboratory experiments have shown the electrical conductivity (σ) of methane hydrate at 0°C to be 5 × 10−5 S/m;
for ice it is of the order of 10−5 to 10−6 S/m. The resistivity of hydrate-saturated sediments approximately varies from 103 to 106 ohm-m (Tzirita, 1992), whereas water has a resistivity of 10 ohm-m and
for sand it is of the order of 102 to 103 ohm-m. The significant contrast in electric parameters due to
the presence of hydrates in sediments make electromagnetic (EM) surveys a good tool to map hydrate
occurrences (Du Frane et al., 2011). The unconsolidated sediments in the upper several hundred meters
of the marine sediment section (50% porosity) normally have a very low resistivity of about 10 ohm-m.
For 15–20% hydrate saturation in the pore space (7–10% of sediment), the resistivity increases by a
factor of about 2. Calculation of hydrate content using downhole electrical measurements based on
Archie’s law requires knowledge of the saturation exponent. The saturation exponent is an empirical
parameter that includes influences from the internal rock structure such as pore shape, size, connectivity of the pore network, and the distribution of the conducting phase. Spangenberg (2004) reported that,
for the parameters used in his model calculations, the saturation exponent varied between 0.5 and 4.
The saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape, size, connectivity of the pore network, and the distribution of the conducting
phase. Seismic measures can delineate gas hydrates in the sediments provided their saturation in the
pore spaces is of the order of ∼40%, as such percentage rises can significantly alter the seismic
9.3 Decomposition or Dissociation
293
velocities to cause anomalous behavior. With lower saturations of gas hydrates in pore spaces, seismic
analysis may not have a greater advantage. This means that some seismic techniques may miss a significant amount of methane hydrate in areas where the saturation is less than ∼40%. As the lack of
appreciable impedance contrast has not been clearly brought out in many seismic studies, apart from
the top of the hydrated zone, the resource estimate of gas hydrate is speculative. Laboratory studies
show that electrical methods are more sensitive to lower saturations of gas hydrates and may also delineate the top of the hydrated layer. This has fueled interest in the application of electromagnetic (EM)
methods for regional characterization of gas hydrate deposits or the joint application of EM and seismic
techniques. The sensitivity of electrical properties to a wide range of hydrate saturations is also manifest by the widespread reliance of borehole resistivity logging to identify hydrate-bearing sediments in
both marine and permafrost associated settings.
Well-planned strategies and technological advancement are the prerequisites for exploitation of gas
hydrates for the production of methane and its recovery in commercial quantities, taking into consideration the likely geological hazards one may encounter during such operations.
9.3 DECOMPOSITION OR DISSOCIATION
Gas hydrates are not chemical compounds as the sequestered molecules are never bound to the lattice.
The formation and decomposition of hydrates are first order phase transitions, not chemical reactions
(Maslin et al., 2010). Hydrate decomposition is a sequence of lattice destruction and gas desorption
processes. The formation and decomposition of gas hydrates is complex and not fully understood
(Bishnoi and Natarajan, 1996; Gao et al., 2005). The stability conditions are likely to be altered with
the presence of varying ionic content of water and the admixture of different constituent gases (Fig. 9.3).
The presence of gases other than methane in the hydrate frame significantly alters the stability conditions. Hydrates stabilize at appreciably higher temperatures and lower pressures when hydrogen sulfide
FIGURE 9.3
Gas hydrate stability conditions. (a) The hydrate stability zone in subsea sediments (source: http://www.pet.
hw.ac.uk/research/hydrate/hydrates_where.cfm). (b) Temperature versus pressure plot for different gases
shows the variation of stability conditions (source: knol.google.com/k/gas-hydrates).
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(H2S), carbon dioxide (CO2), ethane (C2H6), and propane (C3H8) are part of the hydrate-forming
gas mixture (Carroll and Mather, 1991; Sloan, 2006; Kvenvolden, 1998). The presence of hydrogen
sulfide (H2S) and carbon dioxide gases with methane make the hydrate phase equilibrium stable at
higher temperatures and lower pressures. On the contrary, the presence of nitrogen with methane may
cause hydrate formation at higher pressures and lower temperatures.
Hydrates stiffen the sediment in which they exist, and this semiconsolidated mass overlies the
highly fluidized and frequently gassy silts and muds at the base of the gas hydrate stability zone
(GHSZ). Thus, as a general principle, either creation or dissociation of hydrate can potentially cause
problems, though dissociation is the more immediate concern. Dissociation of the hydrate induces fluid
and gas production, loss of strength and stiffness, and elevated pore pressures (Grozic, 2010). Factors
affecting gas hydrate decomposition in sediments include sediment type, mineral composition, pore
size distribution, particle size, pore water composition, hydrate saturation distribution, initial formation
pressure, and temperature and cement characteristics. Furthermore, the decomposition rate of hydrate
sediments decreases with decreasing particle size and increasing pore size (Yu et al., 2013). Where
hydrate exists near the bonds of stability, even small increases in temperature may destabilize it; absorbing heat during decomposition of hydrates can give rise to formation temperature changes. However,
the formation temperature change will limit the decomposition rate of hydrates (Cheng and Li, 2012).
Hydrate dissociation starts when the pressure−temperature (PT) state reaches the equilibrium boundary. In reality, the hydrate stability curve should be expressed as a broad band that reflects different
boundaries for different pore sizes rather than a single phase boundary. A single boundary causes
instantaneous dissociation as temperature increases; in contrast a broad margin across the PT boundary
is expected to cause gradual dissociation. This is due to hydrates in smaller pores dissociating at lower
temperatures compared to those in larger pores (Anderson et al., 2003; Kwon et al., 2008).
Any variations of these factors alter the stability conditions and gas hydrate may decompose and liquefy to produce liquid and large quantities of methane gas. Gases are highly concentrated in the solid
hydrate phase, thus the decomposition of hydrates can release large volumes of gas. One cubic meter of
methane hydrate can contain the equivalent of 164 m3 of methane at standard temperature and pressure
(Sloan, 2006). However, the immediate volume increase that occurs on hydrate decomposition is dependent on temperature and pressure conditions in the sediment column. The volume expansion factor
increases as water depth (hydrostatic pressure) decreases. For example, at a water depth of 1000 m (and
typical seafloor temperatures), the volume of released methane and water is a little over twice the original
gas hydrate volume, whereas at 500 m water depth, the volume increases by a factor of 3.5, and in very
shallow hydrate deposits (<300 m water depth), expansion factors may exceed by a factor of 6.0 (Fig. 9.4).
When solid hydrate turns into gas and water, the volume of pore-filling material can increase significantly and the sediment becomes fluidized, compromising the strength and stiffness of the sediment
column (Birchwood et al., 2010). Strengthening effects of the hydrate are not only lost owing to loss of
material and cohesion but this weakening effect is in addition to the pore pressure effect due to the
release of a high quantum of gas. These many properties and effects, in addition to those introduced by
the environmental pore pressure conditions, place many limits on the conditions under which gas
hydrates may be stable in nature (Durham et al., 2005). Modeling of a gas hydrate system during tectonic uplift revealed that significant pore pressure may build up beneath shallow gas hydrates easily
reaching lithostatic pressure and thus may lead to fracturing of the seafloor.
The presence of free gas in the GHSZ leads to two different problems with respect to hydrate thermodynamic stability. First, it reduces the lithostatic pressure by decreasing the density of the sediment
9.4 Hydrate Impact on Climate
295
FIGURE 9.4
Thickness of the zone of gas hydrate stability in the seafloor (assuming a geothermal gradient of 30°C/km)
depending on water depth (bold line). The bar charts show the equally depth-dependent increase in volume
(number = factor by which the volume increases) if a given volume of gas hydrate dissociates. (From Paull
et al., 2000.)
column. Secondly, and probably more importantly, it steepens the geothermal gradient due to its small
thermal conductivity. As the seafloor constitutes a nearby heat reservoir of essentially infinite capacity,
steepening the gradient causes the temperature at the base of the GHSZ to increase. Both pressure and
temperature effects tend to destabilize the hydrate deposit and this effect in the sediment can be exacerbated by a pore-filling growth habit (Kleinberg et al., 2003).
Accumulation of gas under the base of the GHSZ due to the destabilization of hydrate or migration
from below is also clearly problematic. Increasing gas saturation reduces the shear strength of the formation. With a huge quantum of methane gas preserved in hydrate or free gas zone in shallow subsurface earth under delicate PT conditions, its dissociation may cause a significant geological hazard in
terms of climatic impact and margin slope stability during conventional oil and gas drilling and production operations. In this chapter, we mainly discuss the latter aspect of geological hazards due to the
dissociation of gas hydrates in the continental margins.
9.4 HYDRATE IMPACT ON CLIMATE
Gas hydrates in the sediments are in a state of metastable equilibrium with the environment. Methane
as a guest molecule is captive in a structural frame of hydrates and provides stability to hydrates over a
higher range of temperatures. PT regimes make them stable in shallow subsurface depths of the earth.
Changes brought about in temperature and pressure by any mechanism leads to destabilizing or stabilizing of hydrates over the timescale to the order of hundreds to thousands of years. The stabilizing or
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destabilizing timescales depend on the thermal properties of sediments (MacDonald, 1990). The
amount of methane trapped in the hydrates is almost 3000 times the quantum of methane in the atmosphere (Kvenvolden, 1998). The methane concentration in the atmosphere is increasing at the rate of
about 1% every year (Watson et al., 1990). Release of large amounts of methane from hydrates either
as unaltered methane or oxidized to carbon dioxide could have a large impact on the composition of
atmospheric gases. Methane releases from hydrates that could be most significant to climate change are
more likely to be chronic in nature. Methane, being radioactively active, is a greenhouse gas and a
major contributor to global climatic change (McDonald, 1990). It is suggested that methane released as
a greenhouse gas from hydrates has a 20 times larger effect on global warming as compared to carbon
dioxide integrated into the atmosphere over the last 100 years (Shine et al., 1990).
The shallow hydrate stability zone makes it vulnerable to near-surface changes. Any changes
brought about in PT conditions on the surface or near surface by means of sea-level fluctuations during
periods of glaciations, atmospheric warming due to climatic change, and tectonic activities in terms of
faulting and earthquakes may lead to disturbances in the stability of hydrates. These phenomena alter
the temperature and/or pressure regime thereby altering the stability conditions. During the global
warming period, the glaciers and ice caps melt; increase water flow in the oceans and polar shelves. The
thermal expansion of oceans also causes rise in sea levels. The rise in sea levels results in an increase
in the hydrostatic pressure and stabilizes hydrates in continental margins and in polar continental
shelves (Kvenvolden, 1998). During global cooling cycles, the whole system reverses. Glaciers and ice
caps grow resulting in a fall in sea levels over the continental shelves reducing the hydrostatic pressure
over the continental shelves. The reduction in hydrostatic pressure due to the sea-level drop of 120 m
during the last Glacial Maximum (∼18,000 years ago) may have resulted in decreasing the hydrate
stability zone by about 20 m, assuming the temperature to be constant (Dillon and Paull, 1983). The
reduction in the thickness of the stability zone depends on the rate of sea-level fall and thermal properties of near-seafloor sediment. A global map of the thickness of the GHSZ under steady-state conditions
for the present-day climate (Fig. 9.5a) shows values of 600−900 m in polar regions due to low bottom
water temperatures (<0°C) and along continental margins where bottom water temperatures are low
and a thick sediment cover exists (Kretschmer et al., 2015). Thinner GHSZs (<150 m) are predicted for
most of the deeper ocean, where the vertical extent is restricted by thin sediment thicknesses. The existence of abundant Early Paleogene marine gas hydrates despite warm deep-ocean temperatures was
studied by Gu et al. (2011). Numerical simulations of gas hydrate accumulations were used at Paleogene seafloor temperatures to show that near-present-day values of gas hydrates could have been hosted
in the Paleogene. A modified methane production profile with respect to depth below the seafloor at
different seafloor temperatures is illustrated in Fig. 9.5b. Changes in the thickness of GHSZ are quite
pronounced for higher geothermal gradients (Fig. 9.5c).
Fluctuation in sea level and changes in thickness of sediment column manifests in hydrate stability,
thereby releasing about 200−400 Gt of carbon in the atmosphere over the time scale 1000s years. The
fall in sea levels also changes the near-surface temperature and warms up the oceanic waters. A few
degrees of warming in the deep ocean can have a significant impact on the stability of the hydrate, and
it is known that the temperature of the deep ocean responds to changes in surface climate, albeit with a
lag of centuries to millennia (Schiermeier, 2008). Different mechanisms have been proposed to explain
the cyclic link between the dissociation of gas hydrates (release of methane) and climate change or
global warming. Nisbet (1990) suggested that the methane released from the continental gas hydrates
has led to a rapid rise in the global temperatures, which in turn may have almost stopped glacial activity
FIGURE 9.5
Effect of change in gas hydrate thickness, global gas hydrate stability zone, and methane production profile
for a model. (a) Global map of the gas hydrate stability zone thickness (GHSZ in m) under present-day climate
conditions (mean 1988–2007). The white shadings indicate areas where GHSZ = 0 m. The dashed contour
line marks the 300 m isobath. (From Kretschmer et al., 2015). (b) Methane production profiles with respect to
depth below the seafloor at different seafloor temperatures. Dsf = 2.0 km. The effects of higher organic carbon
input and elevated methanogenesis rates at higher sediment temperatures have been included. The black
bars show the base of the GHSZ for each Tsf °C. (c) Change in thickness and ocean depth effect on temperature
for gas hydrate stability zone.
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about 13.5 ka ago. Termination of a glacial period makes higher latitude regions get exposed to higher
temperatures and results in melting ice sheets or caps and thereby reduces pressure. The change in
thermobaric conditions leads to destabilization of gas hydrates and the release of uncontrolled methane
into the atmosphere.
Conversely, Paull et al. (1991) postulated that during periods of global cooling, water inputs over the
outer continental margins are curtailed causing sea levels to fall. These falling sea levels reduce the pressure over the margins leading to dissociation of gas hydrates and the release of methane into the atmosphere, resulting in global warming triggering deglaciation. Both the proposed models suggest the
interconnectivity between different regions (high latitude − tropical) of gas hydrate occurrences through
the atmosphere. The two proposed models either due to release of methane by continental hydrates, altering global temperatures or global cooling during periods of glaciations over the polar regions and determining the fate hydrates in tropical gas hydrates, envisages interconnectivity between different regions
(high latitude-tropical) through the atmosphere in relevance to gas hydrate to gas hydrates occurrences.
Physical changes brought about in one region lead to perturbing the physical conditions in another region
via atmospheric connectivity. According to the latest discussion by the US Geological Society
(USGS),which refers to the numbered type locals or sectors shown in Fig. 9.6a of gas hydrate deposits,
currently, gas hydrates are most likely dissociating in shallow arctic shelves (sector 2 in Fig. 9.6a) and the
upper edge of stability (sector 3 in Fig. 9.6a), contributing to release of methane into the atmosphere. As
per the USGS (www.USGS.gov), these sectors are as follows:
•Thick onshore permafrost. Gas hydrates that occur within or beneath thick terrestrial permafrost
will remain largely stable even if climate warming lasts hundreds of years. Over thousands of
years, warming could cause gas hydrates at the top of the stability zone, about 625 ft (190 m)
below the earth’s surface, to begin to dissociate.
•Shallow arctic shelf. The shallow water continental shelves that circle parts of the Arctic Ocean
were formed when sea-level rise during the past 10,000 years inundated permafrost that was at the
coastline. Subsea permafrost is thawing beneath these continental shelves, and associated methane
hydrates are likely dissociating now.
•Upper edge of stability. Gas hydrates on upper continental slopes, beneath 1000 to 1600 ft
(300 to 500 m) of water, lie at the shallowest water depth for which methane hydrates are stable.
•Deepwater. Most of the earth’s gas hydrates, about 95%, occur in water depths greater than
3000 ft (1000 m). They are likely to remain stable even with a sustained increase in bottom
temperatures over thousands of years.
An example of methane bubble plumes off Washington, Oregon suggests warmer ocean may be
releasing frozen methane (www.phy.org) as shown in Fig. 9.6b. New University of Washington research
suggests that subsurface warming could be causing more methane gas to bubble up off the Washington
and Oregon coast. However, due to the long ventilation times of the deep ocean (∼100–1000 years),
where the bulk of methane hydrates reside, and the slow propagation of the temperature signal into the
sediment column (∼180 m/1000 years), a new equilibrium is only reached on a timescale of
1000–10,000 years. Moreover, the fraction of methane from the bottom of the ocean that reaches the
atmosphere is uncertain and dependent on the transport mechanism, e.g., bubbles, dissolved (Lamarque
2008). However, the susceptibility of hydrate dissociation to near-surface natural or human-induced
changes in pressure and temperature might have played an important role in past climates and may also
bring about significant global climatic change in the future due to activities for extraction of gas
9.4 Hydrate Impact on Climate
299
FIGURE 9.6
Gas hydrate deposit over continental margins and gas bubble rising. (a) Gas hydrate deposits by sector.
Currently, gas hydrates are most likely dissociating in sectors 2 and 3. Only sector 2 is likely to release methane
that could reach the atmosphere (source: http://soundwaves.usgs.gov/2012/06/). (b) Sonar image of bubbles
rising from the seafloor off the Washington coast. The base of the column is 1/3 of a mile (515 m) deep and
the top of the plume is at 1/10 of a mile (180 m) depth. (Credit: Brendan Philip/University of Washington.)
hydrates as an energy resource. Therefore, there is an increasing interest in assessing the potential of
methane release in a warmer world and its consequences for future climate change (Brook et al., 2008;
Schiermeier, 2008; Westbrook et al., 2009).
Interpretation of identified BSR in seismic reflection data suggests that the causative mechanism of
BSR requires free gas, trapped-gas as bubbles in interconnected voids in sediments. The thickness of
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the free gas zone and the quantum of methane preserved in this zone are still speculative. In particular,
the size and role of the free gas zone below the gas hydrate provinces remain relatively unconstrained,
largely because the base of the free gas zone is not a phase boundary and has thus defied systematic
description. It is suggested that the maximum thickness of an interconnected free gas zone is mechanically regulated by valving caused by fault slip in overlying sediments as seen in Fig. 9.7. Modeling
results (Hornbach et al., 2004) show that such critical gas columns exist below most hydrate provinces
in basin settings, likely poised for mechanical failure and are therefore highly sensitive to changes in
ambient physical conditions. These studies indicate that the estimated global free gas reservoir may
contain from one-sixth to two-thirds of the total methane trapped in hydrate. If gas accumulations are
critically thick along passive continental slopes, a 5°C temperature increase at the seafloor could result
in a release of ∼2000 Gt of methane from the free gas zone, offering a mechanism for rapid methane
release during global warming events (Hornbach et al., 2004).
9.5 GEOLOGICAL HAZARDS
Tectonic forces and sea-level changes have brought in many changes in seafloor morphology by
deforming the soft unconsolidated sediments on the continental margins. The shape and scale length of
these features are governed by smoothness and gradients in the seafloor relief (Fig. 9.8). Over the continental shelf or slope regions, under normal circumstances these slumps are gravitationally driven
when the mechanical shear strength of sediments does not overcome the gravitational force. Gravitydriven sediment mass transport can be classified as slide slump and debris flow. Movement of a large
coherent mass of sediments with no internal deformation over the planar glide plane in contact bedding
is termed as slide. The slump occurs as a coherent rotational mass movement on the concave up glide
plane with internal deformation of the slumped mass. Debris flows are incoherent viscous masses in
which intragranular movement produces a chaotic end product. Increase in water content makes the
debris flow into turbidity currents (Shanmugam et al., 1995). The gravity-driven mass transport of sediments on the continental slope has quite distinct characteristics: slides occur on large planar surfaces,
no internal deformation is seen, and the end member is debris flow.
When the seafloor surface is inclined, the gravitational force tends to move the sediment mass
downward. This movement is resisted by the shear strength of sediments along the rupture surface.
When the gravitational force exceeds the resistance offered by the sediment strength, slope failure is
likely to occur (Siriwardane and Smith, 2006). Sub-aerial and submarine landslides occur when the
stresses exerted on the sediments exceed the sediment strength. Increase in shear stress or decrease in
shear strength can lead to shear failure in sediments, which in turn leads to slope failure. These stresses
are from external sources or from changes brought about within the sediment matrix itself. In the
marine environment, slope failure may result from over-steepening of slopes, faster sediment deposition, tectonics, and earthquakes. Submarine gas hydrate deposits are found on continental slopes, where
average slope grades are only a few degrees. Basic fracture mechanics theory suggests that low angle
failures cannot result from slow changes of earth stresses or mechanical properties. Low angle failures,
including those found in geological records, can only result from (1) large transient stresses, such as
those associated with earthquakes, (2) sudden changes in material properties, such as those associated
with rapid decomposition of gas hydrate, or (3) sudden changes of pore pressure, which might be due
to hydrate decomposition or migration of gas from other sources (Kleinberg, 2004). The mechanisms
9.5 Geological Hazards
301
FIGURE 9.7
Blue symbols (numbered 1–9) show gas column thickness below BSRs at several hydrate provinces in basin
settings where 1 is near vertical. Red symbols (10–19) show gas column thickness below BSRs at hydrate
provinces in compressional settings where 1 is probably near horizontal. Note that gas column thickness
generally increases with depth for the blue points, but remains nearly constant for the red points. The gray
area indicates the interconnected FGZ thickness required for fault reactivation. The upper bound of the gray
area corresponds to values of α = 0.77 (zero hydrate at the BSR) and υ = 0.45; the lower bound corresponds
to low end values of α = 0.67 (25% bulk hydrate) and υ = 0.41. Point locations: 1, Barents Sea (Andreassen
et al., 1990); 2, the Norwegian margin (Mienert et al., 2000); 3, Bering Sea basin (Scholl and Hart, 1993);
4, 5, Blake Ridge (Holbrook et al., 1996); 6, Beaufort Sea (Andreassen et al., 1995); 7–9, basin in Makran
accretionary prism that has experienced rapid sedimentation (Sain et al., 2000; Grevemeyer et al., 2000); 10,
offshore Cascadia (Hyndman et al., 2001); 11, offshore Chile (Bangs et al., 1993; Miller, et al., 1996), Niger
delta (Hovland et al., 1997); 13, 15, offshore Colombia (Minshull et al., 1994); 14, Gulf of Oman anticline
(Minshull and White, 1989); 16, offshore Peru (Pecher et al., 1996); 17, South Shetland islands (Tinivella
et al., 1998); 18, 19, Oman accretionary toe (White, 1979). Top inset, model of the hydrate stability zone and
critically thick FGZ below for a bottom water temperature of 0°C. The BSR is located at the interface between
the sediment overburden where hydrate can exist (green) and the FGZ (black and white stripes). The pink
area marks the hydrate wedge. Bottom inset, new BSR depth and critical FGZ thickness for a 5°C temperature
rise at the seafloor. The gray area denotes the area over which gas (and dissociated hydrate) must escape.
Note the thinner critical FGZ due to bottom water warming, which results in less sediment overburden above
the BSR. (From Matthew et al., 2004.)
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CHAPTER 9 GEOLOGICAL HAZARDS
FIGURE 9.8
(a) Schematic diagram showing four common types of gravity-driven downslope processes (slides,
slumps, debris flows, and turbidity currents) that transport sediment into deep-marine environments
(from Shanmugam et al., 1994).(b) Sediment concentration (% by volume) in gravity-driven processes.
Note that turbidity currents are low in sediment concentration (i.e., low-density flows) (after Shanmugam,
2000). (c) Based on mechanical behavior of gravity-driven downslope processes, mass-transport processes are considered to include slide, slump, and debris flow, but not turbidity currents (Dott, 1963).
(d) The prefix “sandy” is used for mass-transport deposits (SMTDs) that have grain (>0.06 mm: sand
and gravel) concentration value equal to or above 20% by volume. The 20% value is adopted from the
original field classification of sedimentary rocks by Krynine (1948).
that potentially cause shear failure also include (a) removal of lateral supports, (b) increase in the
weight of overlying sediment, (c) upward fluid flow (Davies and Clark, 2006), and (d) frictional heat
generated in the subduction zone (Jahren et al., 2005).
Geological hazards produced by gas hydrates are related to the instability of the sediment pile due
to the decomposition of hydrates or possible violent releases of methane in the water column, and further in the atmosphere. The decomposition of hydrates has to be rapid enough in order to trigger
hazard-producing phenomena. At geological scale, “rapid enough” means dynamics of the decomposition
processes able to suddenly influence the consolidation process of marine sediments. For causing deformation on the scale length of slope failure, gas hydrate accumulation has to be widespread, the slide has
to occur in the hydrate stability zone, and there has to be reduction in permeability at the base hydrate
stability zone to accentuate pore pressure, which may lead to slope instability due to any change in
external physical conditions (Dillon and Max, 2003).
Parameters affecting gas hydrate formation include temperature, pore pressure, gas chemistry, and
pore water salinity. During gas hydrate formation, methane and water become immobilized as a solid,
restricting pore space and retarding the migration of fluids. Hydrates occupy pore spaces and inhibit consolidation due to sedimentation and mineral cementation. However, hydrates themselves can act as metastable cementation for sediment grains. The continuous process of hydrate formation impedes the
permeability of the sediment as gases and liquids combine to form hydrates. Eventually, gas hydrate may
9.5 Geological Hazards
303
FIGURE 9.9
Champagne cork effect: illustration of what happens when a slope failure occurs above a gas hydrate layer
and how large quantities of gas can be released. (From http://rsta.royalsocietypublishing.org/content/368/1919
/2369Kvenvolden, 1998.)
occupy much of the pore space within the GHSZ. Continued sedimentation leads to deeper burial of the
gas hydrate. Finally, the gas hydrate is buried so deep that temperatures at the base of the stability zone
are those at which the gas hydrate is no longer stable. Thus, the basal zone of the gas hydrate becomes
underconsolidated and possibly overpressured because of the large quantum of released gas, leading to a
zone of weakness. This volume expansion of gases causes the pore fluids to become highly pressurized
and push the sediment apart resulting in the loss of sediment compaction, formation of large pore spaces
leading to fracturing in sediments, and potentially triggering submarine landslides. Failure follows on
moderate slopes unless the increased fluid pressures can be adequately vented (Kvenvolden, 1999).
Submarine landslide phenomena and gas hydrate dissociation is a linked, auto-feedback process (Fig.
9.9). A landslide may be triggered either by gravitational or sea-level changes, tectonic or earthquake
activities, or due to processes over a small segment of continental slope, which may be under the gas
hydrate-bearing layer. Transportation of sediment mass in the upper sediment layer lowers the lithostatic
pressure over the hydrate layer, thereby disturbing the hydrate stability equilibrium. During decomposition,
the mechanical strength of sediments is considerably reduced and the sediments can no longer support the
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CHAPTER 9 GEOLOGICAL HAZARDS
load of the overlying strata. Destabilizing the gas hydrate will release a large quantum of gas and liquid and
cause an increase in pore pressure, decrease in shear stress, and loss of shear strain of the sediment layer by
increasing the pore size. Reduction of sediment strength and thinning of the hydrated layer makes the upper
layer slide further down along the slope due to gravity and release more gases and fluids (McIver, 1982).
Near-surface changes in PT conditions lead to the destabilization of gas hydrates. Over the continental margins, pressure being the dominant component regulates the thickness of the GHSZ, thickening with increase in the level of the water column and in its reduction during sea-level fall. When gas
hydrates dissociate due to any of the aforementioned causes, the released methane either gets partially
dissolved in the overlying seawater or can also remain in gaseous phase with the seawater. The dissolution of methane in seawater reduces the density of seawater (Xu and Ruppel, 1999), leading to mass
transfer, which is a slow process (Zhang and Xu, 2003). The estimated values of methane release due
to the dissociation of unit volume of gas hydrate in an oceanic regime exhibit a distinct trend (Max and
Dillon, 1998; Paull et al., 2003). The pore pressure on sediments that are close to the up-dip end of the
GHSZ are significantly higher when compared to deeper parts of the ocean resulting in larger quantities
of methane release near the up-dip side in the shallow regions, leading to a reduction in sediment
strength. The exposure of hydrated sediments to warmer ocean conditions, higher gradients on the
upper side of the slope, and the tendency of free gas to move to the flanks weaken the sediment strength
and make them susceptible to sliding (Dillon and Max, 2003).
Gas hydrates occupy pore spaces and also cement sediments to increase the shear strength and
enhance the slope stability. On the one hand, hydrates may strengthen the sediments by cementing
grains; on the other hand, if hydrates impede fluid flow due to reduced permeability, they may weaken
the underlying sediment by trapping fluids and free gas. Coarse-grained sediments are favorable for gas
hydrate growth but impede the bulk permeability of the host sediments. The internal mechanisms which
weaken the sediment strength are related to increased porosity, pore pressure, and the presence of gas
bubbles in the pores. Any causative mechanism that alters the stability equilibrium may result in dissociation (gas hydrate turns into free gas or water mixture) and/or dissolution (gas hydrate becomes a
mixture of water and dissolved gas) of the gas hydrate. During the formation of gas hydrates, the water
and the gas content in the sediment pores is converted to solid form and increases sediment strength.
On the contrary, the dissociation of gas hydrate will freshen the sediment with water and gases, which
may reduce the sediment strength. The generation of excess pore pressure due to hydrate dissociation
is known to be the most relevant process responsible for destabilizing hydrate-bearing sediments (Xu
and Germanovich, 2006; Moridis and Kowalsky, 2007). The excess pore pressure reduces the effective
stress of sediments and the shear strength, which results in shear failure. The excess pore pressure generated by decomposition of gas hydrates depends on the hydrate saturation in the pore, the gas solubility, and medium compressibility (Sultan et al., 2004) and the pore pressure increases with temperature.
The magnitude of excess pore pressure depends on the initial methane concentration and hydrostatic
pressure. For a hydrostatic pressure of 4 MPa, the dissociation of hydrate will generate a pore pressure
of 0.1 MPa for the initial hydrate saturation of 2.6% and pore pressure increases to 0.3 MPa for the
hydrate concentration of 52% in the pore spaces as seen in Fig. 9.10 (Sultan et al., 2004).
Gas hydrate decomposition at the upper shelf edge can result in an enormous pore pressure that leads
to a massive loss of compactness while the large pore space makes the sediment highly deformable. As
would be expected, the steeper the sediment slope, the higher the probability that a slide will occur,
where slides are more likely to be promoted by gas hydrate breakdown on slopes of more than 4°. Laboratory experiments, small-scale physical modeling, and theoretical slope stability analyses indicate that
dissociation of even a small amount of hydrate can cause a significant loss of sediment strength. Hydrate
9.5 Geological Hazards
305
FIGURE 9.10
Excess pore pressure generated by the melting of methane hydrate at different gas hydrate pore saturations.
For a hydrostatic pressure of 4 MPa, the melting of the methane hydrate will generate around 0.3 MPa for an
initial hydrate fraction of 52%. (From Sultan et al., 2004.)
dissociation could be critical in inciting slope failures for low-permeability sediments in shallower water
depths (Nixon and Grozic, 2006). Based on the characteristics of the medium, the pore pressure response
differs significantly. The dissipation pore pressure in clayey rocks is not appreciable, which makes this
medium more susceptible to deformation and possible slope failure. In sandy rocks, the pore pressure
dissipates faster, although pore pressure buildup is rather slow (Kayen and Lee, 2001). In coarse-grained
rocks, pore pressure in conjunction with other external factors such as earthquakes and faster rate of
sedimentation may lead to seafloor collapse or failure (Kayen and Lee, 1991). The most important factor
for controlling pore pressure generation is hydraulic permeability. Generation of excess pore pressure is
a function of the amount of gas hydrate present initially, the rate of gas hydrate dissociation, sediment
permeability, background fluid flux, capillary pressure, and the depth of the base of the GHSZ
(Crutchley et al., 2010; Nimblett and Ruppel, 2003). Fine-grained sediments are more susceptible to
hydraulic fracturing due to the high gas pressure needed to overcome capillary entry pressure. This process reopens the fluid pathway and locally increases the bulk permeability of the sediments (Scholz
et al., 2011). A number of oceanic gas hydrate deposits such as the Ulleung Basin sediments (Kwon
et al., 2011), hydrate deposits in the Gulf of Mexico (Francisca et al., 2005), and the Krishna–Godavari
basin in India (Yun et al., 2010) are characterized by fine-grained low-permeability sediments.
The top of the GHSZ is generally a diffused (hydrate or seawater) boundary due to a lack of noticeable contrast in physical properties and is therefore difficult to identify in geophysical field datasets.
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CHAPTER 9 GEOLOGICAL HAZARDS
Gas hydrate dissociation at the base of the GHSZ is often considered a major cause of sediment deformation and submarine slope failures. Based on theoretical considerations it is suggested that excess
pore pressure and shear discontinuities generated by hydrate dissociation are unlikely to be a hazardous
factor. In the natural environment, the excess pore pressure generated by hydrate dissociation is bound
by the gas hydrate stability law inducing a natural temperature increase, a limited amount of excess
pore pressure, and limited shear discontinuities at the base of the GHSZ. However, under natural temperature changes hydrate dissolution at the top of the GHSZ, (which can occur at a regional scale) is a
hazardous process that can lead to catastrophic landslides (Sultan et al., 2004). During the process of
hydrate dissociation, gas hydrates liquefy and sediments unconsolidate. The free-gases trapped beneath
the remaining gas hydrates would constitute a weak layer of overpressurized sediments. The combined
effect of these processes may lead to slope failure (Bünz et al., 2003).
Periodic Pleistocene eustatic sea-level transgressions and regressions provide a mechanism to
account for the waxing and waning of submarine gas hydrates. Sea-level changes, especially the lowering of sea levels during the falls, have given rise to large-scale slumping along the margins. Fluctuations in global climate, reflected in Pleistocene sea-level falls, likely caused these submarine slides and
perhaps caused other slides on other continental margins where gas hydrates are present (Kayen and
Lee, 1991).
During the last glacial period between 45 ka and 16 ka about nine continental slope failures confined to the Heinrich event are reported in the middle and low latitudes, and these failures were supposedly triggered due to lowering of the sea levels. About 70% of the slope failure occurred during the past
45 ky was displaced in two periods, between 15 and 13 ka and between 11 and 8 ka. Both these intervals correlate with rising sea levels and peaks in the methane record during the Bølling-Aollerød and
Preboreal periods (Fig. 9.11).
The glaciation and deglaciation period also changes the thermal pattern of continental margins.
During the period of lower sea levels, hydrates are exposed to warmer surface temperatures. Also the
global scale current patterns in the oceans periodically alter the sub-bottom temperature of the oceans.
This is an important consideration as it is the intermediate ocean waters between about 200 and 1500 m
which bathe the sediment most likely to contain gas hydrates. This predicted warming of the intermediate ocean depth tends to destabilize marine gas hydrates. The small predicted rise of future sea levels
of up to 1 m may increase the hydrate stability slightly but is completely insufficient to counter the
FIGURE 9.11
Methane record during the Bølling-Aollerød (BA) and Preboreal (PB) periods.
9.5 Geological Hazards
307
warming. However, to destabilize gas hydrates through increased temperatures, the increased warmth
must penetrate through the gas hydrate layer to the phase transition at the hydrate–gas boundary. Thermal diffusivity is a relatively slow process and related to the temperature gradient, the hydrate depth,
and sediment composition (Clennell et al., 1999; Henry et al., 1999).
Paleocene−Eocene thermal maximum (PETM) is the most intensely studied high temperature event
that occurred ∼54.95 Ma. The large, negative carbon isotopic excursion (CIE) recorded in both marine
and terrestrial sediments during the PETM has been interpreted as reflecting widespread release of
isotopically-light (microbial) carbon from dissociating marine methane hydrates (Dickens et al., 1995;
Zachos et al., 2005). It has also seen a number of climate and carbon cycle modeling studies attempting
to integrate the available proxy data into a coherent view of the earth-system behavior across the event
(e.g., Bice and Marotzke, 2002; Panchuk et al., 2008; Zeebe et al., 2009). The current synthesis
(although with significant continuing uncertainties) appears to consist of an approximately 4% negative
CIE in the oceanic dissolved inorganic carbon pool, a global temperature anomaly of 5–6°C, a rapid
(less than 10 ky) onset, a total PETM duration of approximately 170 ky, and a carbon input within the
range of 4000–7000 Gt with some contribution but perhaps not the dominant one, from methane
hydrates as shown in Fig. 9.12 (Shipboard Scientific Party, 2002).
It has been reported that during the past 55 years (1955–2010) the 0–2000 m layer of oceans worldwide has been warmed by 0.09°C because of global warming. The global warming has latitude dependence in the increase of temperature, more pronounced over higher latitudes when compared to lower
latitude regions. This raises the scientific concern that if warming of the bottom water of deep oceans
continues, it would dissociate natural gas hydrates and could eventually trigger massive slope failures
(Kwon and Cho, 2012). For example, a 1°C increase in the temperature of hydrate-bearing sediments
results in the pore fluid pressure increasing by the order of several megapascals under the no mass flux
condition (Jang and Santamarina, 2011; Holtzman and Juanes, 2011). Therefore, any thermal change
can stimulate hydrate dissociation and thus mechanically destabilize sediments.
The presence of BSR, in the vicinity of deformed structural configuration, has led to the inference
that many submarine slope failures are due to reduction in the shear strength. Seafloor slope failures
occur in the hydrate-bearing sedimentary deposits and might be directly triggered, or at least primed by
gas hydrate decomposition. Submarine slope failures have been reported from many regions of the
world and the different causative mechanisms have been proposed for such features (Kayen and Lee,
1991; Maslin et al., 1998; Rothwell et al., 1998; Dillon et al., 2001; Holbrook, 2001;Vogt and Jung,
2002; Canals et al., 2004; Kwon and Cho, 2012). For almost three decades, the link between gas
hydrates and slope failure has been examined and debated extensively. Most of the studies have
attempted to link gas hydrate decomposition and slope failures. The inference about connectivity
between the gas hydrate and slope failure is drawn on the basis of identification of slope failure in the
vicinity of occurrence of gas hydrate-bearing sediments either by means of the presence of BSRs or
other geophysical, geochemical, and geological signatures (Summerhayes et al., 1979; McIver, 1982;
Nisbet and Piper, 1998; and others).
9.5.1 STORREGA SLIDE-NORWEGIAN SEA
The Storrega Slide in the Norwegian Sea, Norway, is one the most impressive scars preserved on the
continental slope; it is dated at around 7000 years old. The scar is about 290 km long and is comprised
of three segments. More than 5500 km3 of its sediments have been eroded during their transportation
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CHAPTER 9 GEOLOGICAL HAZARDS
FIGURE 9.12
A stacked record of temperatures and ice volume in the deep ocean through the Mesozoic and Cenozoic periods. LPTM, Paleocene−Eocene thermal maximum; OAEs, oceanic anoxic events; MME, Mid-Maastrichtian event
over 800 km towards the Atlantic Ocean (Bugge et al., 1987; Paull et al., 2000; Berndt et al., 2002).
The Storegga Slide is bound west-north-west by the Voring Plateau, further northwest by the Norwegian basin, and in the south by North Sea Fan (Fig. 9.13).
This massive slide is a compound of three events. Recent studies indicate that all these events
took place at ∼8.15 ka (Hafidason et al., 2001). The headwall of the slide is approximately 300 km
9.5 Geological Hazards
309
FIGURE 9.13
Bathymetry of the Storegga Slide, Norway illustrating the enormous size of mass-transport processes. Right:
modeling of the tsunami that would be caused if a similar but one-third smaller landslide than the Storegga
Slide happened in the Fram Strait. (From Berndt et al., 2009.)
wide and the slide narrows to a 60-km gateway in the opening between the Møre and Vøring volcanic highs (Bryn et al., 2005). The slide runs from the shallow depths of the shelf of Norway to the
abyssal plain with a slope angle of 0.6°; major areas of the slide have an inclination <0.4°
(Kvalstad et al., 2002). The genesis of the Storegga Slide is debated extensively and consensus about
the mechanism for its formation has not emerged so far (Bugge et al., 1987; Berndt et al., 2002;
Maslin et al., 2004; Sultan et al., 2004; Mienert et al., 2005; Kvalstad et al., 2002, 2005; Bryn et al.,
2005). In general, excess pore pressure is considered the main causative mechanism for the genesis
of the slide; however, the interpretation for the generation of excess pore pressure differs significantly. This region has experienced seismic activity in the past and is disposed to earthquakes, the
most recent being in 1998.
Two seismic profiles were shot across the Storegga Slide and the data obtained indicated that the
headwall slide is characterized by rollover structures under the undisturbed sediment section, suggesting that the mass has been transported from the base of rollover structures. The local extent of disturbance in the sediment pattern is confined to the fault region which rules out the possibility of
earthquake-generated liquefaction to account for the observed sediment pattern. The identification of a
BSR (Fig. 9.14) in the vicinity lent credence to the dissociation of gas hydrate as the most likely mechanism for the Storegga Slide (Berndt et al., 2002). The Storegga composite slump scar runs to about 290
km with run off distance of about 800 km (Fig. 9.14). Seafloor collapse within the Storegga Slide and
above the dissolved gas hydrate suggests that the hydrate reservoir is highly dynamic, and methane
released due to landslide may contribute significantly to a greenhouse effect (Berndt et al., 2002). A
large amount of methane is released into the atmosphere as a result of submarine sediment failures. A
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CHAPTER 9 GEOLOGICAL HAZARDS
(a)
(b)
(c)
FIGURE 9.14
Migrated high-resolution seismic data. (a) Multichannel data. (b,c) Single-channel data. Note that the bottomsimulating reflector (BSR) within the slide area in (c) mimics the seafloor indicating that it has adapted to the
new pressure/temperature conditions. (From Berndt et al., 2002)
9.5 Geological Hazards
311
link between releases of methane due to dissociation of hydrate and its climatic impact can be ascertained through the clathrate gun hypothesis. More than 70% of slope failures in the North Atlantic
occurred during two time spans, between 15 to 13 ka and between 11 to 8 ka, and correlate with rising
sea levels and peaks in the methane record during the Bølling-Ållerød and Preboreal periods. These
data support the clathrate gun hypothesis for glacial−interglacial transitions (Maslin et al., 2004).
Sea-level changes associated with glaciation and interglaciations have been attributed to many slope
failures across the globe due to variations in PT on the earth’s surface. The last episode of this activity
was confined to the Quaternary era and most of the slope failure was found to have occurred during the
Pleistocene−Holocene epoch (Vogt and Jung, 2002). Maslin et al. (2004) have shown a good correlation between slope failure and sea-level changes. These slope failures correlated well with sea-level
rises and falls and enhancement in methane in the northern hemisphere. Most of the slope failures over
the North Atlantic occurred during the Preboreal period. The Storegga Slide and Andoya failures are
the main events that seem to have formed during this period. The isostatic rebound results in the reduction of lithostatic pressure and dissociation of gas hydrates leading to slope failure. Isostatic rebound
also produces a significant increase in seismics and earthquakes, which may also give rise to slope
failure (Mörner, 1991). Such prevailing conditions may have formed the Storegga Slide during the
Preboreal period (Maslin et al., 2004).
Seismic reflection data delineated the BSR in the Storegga Slide area and its vicinity. Relatively faster
increases in temperatures in ocean waters was shown during the Younger Dryas (∼12.5−10 ka) and warmer
ocean temperatures have prevailed since then. Owing to warm ocean water conditions, there was a substantial reduction in the gas hydrate stability zone along the upper slope of the mid-Norwegian margin,
even with the rise in sea-level during this period. Additionally, the BSRs within the slide complex seem to
have more or less adjusted to the new equilibrium conditions, highlighting the dynamics of hydrate stability in continental margin sediments under environmental changes (Mienert et al., 2005).
Sultan et al. (2004) theoretically studied the thermodynamic, chemical equilibrium of gas hydrates
in soil by considering the effects of pressure, temperature, pore pressure, and pore water chemistry. The
modeling results indicated that increase in temperature and pressure results in dissociation at the top
portion of the GHSZ. Modeling of the Storegga Slide under the assumption of unique shear strength
profile and unique gas profile suggests that pressure reduction and temperature increase during deglaciation leads to dissociation of gas hydrates and the origin of retrogressive failure of the Storegga Slide.
Moreover, due to the gas solubility, the failure interface is initiated at the top of the hydrate layer not
the bottom of the hydrate stability zone as proposed earlier by Kvalstad et al. (2002).
Late Jurassic early Cretaceous fault activation and earthquakes generated from the sediment load
and deglaciation add dynamic load and excess pore pressure which may lead to the initiation of slope
failure in marine clayey sediments. The Storegga Slide was most likely triggered by a strong earthquake in the area 150 km from the slide 8200 years ago. Modeling results show that excess pore pressure generated in the North Sea Fan area is transferred to the Storegga area with reduction in the slope
stability in the downslope from the Orman Lange gas field and developed as a retrogressive slide (Bryn
et al., 2005; Kvalstad et al., 2005).
9.5.2 CAPE FEAR SLIDE − ATLANTIC MARGIN
About 200 slide scars have been mapped on one of the most extensive gas hydrate locals on the Atlantic
continental margin of the United States (Booth et al., 1994). The occurrence of slides in the vicinity of
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identified BSRs and other proxies of gas hydrates provided opportunities to validate the relationship
between them (Carpenter, 1981; Cashman and Popenoe, 1985; Rothwell et al., 1998; Paull et al., 2003).
The Cape Fear Slide and the Cape Lookout Slide are two prominent slope failures situated east of Cape
Fear on the Carolina rise off the eastern continental margin of the United States. The amphitheatershaped headwall scarp running 50 km in length is elevated by 120 m at 2600 m water depth on the
continental slope (for location map see Fig. 9.14). Two salt diapirs appear within the slide to disturb the
slope topography as the diapirs protrude above the seafloor. The mass wasting associated with the slope
extends 250 km across the rise and into the Hatteras Abyssal Plain (Embley 1980). The Cape Fear Slide,
the largest known submarine landslide on the U.S. Atlantic continental margin, is a recent (∼20 ka) mass
wasting event that generated potentially large tsunamis.
Multiparameter geophysical surveys along with drilling over the headwall of the slide were
undertaken to ascertain the triggering mechanism of the slide (Paull et al., 1996; Popenoe et al.,
2001; Hornbach et al., 2007a,b). Initially it was proposed that the triggering mechanism may be
linked to the presence of diapirs (Cashman and Popenoe, 1985) and dissociation of gas hydrates
either due to the lowering of hydrostatic pressure during glacial low stands or due to global warming.
The higher thermal conductivity of salt diapirs leads to an increase in the geothermal gradients; thinning the GHSZ over upwarp and sharp geothermal gradients at the flanks of diapir may promote
slope failure.
Identification of BSRs in the Cape Fear Slide area infers the presence of gas hydrates in this
region. Shallow sediment samples collected during the drilling on the headwall of the slide have
provided an estimate of ages. Carbon-14 dating estimates that the ages range from 9 to 14.5 ka suggesting that the Cape Fear Slide may have formed during the last glacial sea-level low stand, which
may have occurred ∼18 ka (Paull et al., 1996). Sediment lithology and accumulation rates in the
cores believed to be continuous over the Carolina Rise have not changed much during sea-level
changes over 35 ka (Haskell, 1991). With the absence of much change in sediment rate pattern and
lower seismicity, the Carolina Rise and Blake Ridge are some of the most prospective gas hydrate
areas of the world.
Owing to the processes of formation and mass movement of the Cape Fear Slide, the thick salt layer
has been displaced and compressed into a line of diapirs. BSRs can easily be identified to the south of
the slide area and the crest of Blake Ridge. A BSR is conspicuously not seen under the slide, suggesting
that saturation of gas hydrates and free gas in the sediments is quite low. Upslope of the lower headwall
scar, a series of slip planes that flatten with depth and stretch into a stratigraphic layer that is characterized by irregular bedding and thickening and thinning suggest zone weak strata. The zone of weakness
coincides with the discontinuous base of the gas hydrate layer (Popenoe et al., 2001).
A significant number of faults have been identified in the region on the seismic reflection data;
these faults appeared to be near the BSR and the headwall of the slide. The base of the gas hydrate
zone, i.e., the BSR, coincides with the plane of slide over which the mass movement may have taken
place. A BSR is conspicuously absent under the slide and the BSR upwarps at the flanks of the slide
and is found to be present beyond the extent of the slide. Absence of the BSR under the slide suggests
dissociation of hydrate and escape of gas resulting in weakening of the sediment strata (Dillon et al.,
1993; Paull et al., 2003). It is hypothesized that gas hydrate dissociation reduces the sediment
strength and increases the chances of continental slope failure. Circumstantial evidence strongly suggests that gas hydrate breakdown is instrumental in triggering sediment mass movements on the
seafloor (Paull et al., 2003).
9.5 Geological Hazards
313
Multibeam and chirp data were collected and processed to produce the highest resolution bathymetric and subsurface images of the Cape Fear Slide. High-resolution images constrain shallow subsurface
structures of the region and a single massive normal fault that intersects the main Cape Fear Slide
headwall was delineated (Hornbach et al., 2007a). The fault strikes parallel to a linear chain of salt
diapirs and extends ∼100 km south of the Cape Fear Slide.
An M7.5 earthquake located up to 102 km from the upper slope could cause failure on slopes of 6°,
but such high slope angles are confined to only small portions of the continental margin (Fig. 9.15).
Failures on the more typical slopes of 2° require the epicenter to be within 62 km of the shelf edge. An
M6.5 earthquake could cause a landslide only if located within 28 km of the continental slope of 2° and
within 42 km of a 6° slope, and an M5.5 only if located within 7 and 14 km, respectively (Brink et al.,
2009). Analysis of the interactions with the gas hydrate phase boundary and the generations of various
FIGURE 9.15
Slope angle of the US Atlantic continental margin and overlays of interpreted slope failures. (From Twichell
et al., 2009.)
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CHAPTER 9 GEOLOGICAL HAZARDS
slides indicates that only the most landward slide likely intersected the phase boundary and inferred
high gas pressures below it. Interpretation of the acquired data suggests slide failure initiated along this
fault. Studies indicate that this fault is active and continuous salt diapirism over this area suggests that
slide failure may likely occur again along the fault. Ongoing salt intrusion activates this fault and augments slope instability in the region. Hydrodynamic modeling suggests that mass wasting events due to
the slide and faulting generates ocean waves which may create large tsunamis that could significantly
impact the US coast (Hornbach et al., 2007b).
Most submarine landslides on the continental margin occur on the continental slope and upper rise;
a lack of detailed maps for parts of this region has hampered efforts to produce a quantitative assessment
of tsunami hazard. During May 11−25, 2009, a team of scientists conducted a 15-day survey aboard the
National Oceanic and Atmospheric Administration (NOAA) ship Ronald H. Brown to provide a complete seafloor map of the continental slope and upper rise from Cape Hatteras in the south to the eastern
end of Georges Bank in the north, a distance of 1200 km (750 miles).
9.5.3 BEAUFORT SEA
The continental shelf of the Beaufort Sea is composed of complex marine and nonmarine sequences of
clay, silt, and sand (Fig. 9.16, Brink, 2009). Seismic surveys show that the area of the continental slope
across which the seismically inferred gas hydrates occur exceeds 7500 km and may encompass a significant part of the Arctic Ocean basin where the water depth is deeper than 400 m to 600 m at different
locations. In many areas of the shelf these sediments contain occurrences of ice-bonded permafrost and
associated pressure and temperature conditions that are conducive to the occurrence of methane gas
hydrates. The most extensive offshore occurrence of gas hydrates in the North American Arctic lies
north of Alaska, beneath the Beaufort Sea (Grantz and Dinter, 1980). The Beaufort Sea continental
slope is disrupted by a belt of massive bedding plane slides and rotational slumps (Kayen and Lee,
1991). USGS seismic surveys have shown clear evidence of landslides on the Beaufort Sea continental
slope at depths between 200−400 and 2000 m. The shelf to slope transition is thought to be an area of
extensive regional instability with acoustic records indicating there is upwards of 500 km of slumps and
glides extending over the entire Beaufort margin. Some of these slide regions are coincident with the
up-dip limit of the permafrost gas hydrate stability (Grozic and Dallimore, 2012). BSRs from the sediment of the Beaufort Sea are most strongly developed beneath the bathymetric highs. The base of the
gas hydrate zone coincides with a region of sediment-­containing gas hydrates.
This complex environment is undergoing dramatic warming, where changes in sea level, ocean
bottom temperatures, and geothermal regimes are inducing permafrost thawing and gas hydrate
decomposition. Eustatic sea-level falls during Pleistocene times caused reduced pressures acting on
the seafloor sediment. In oceanic areas underlain by sediment with gas hydrate, the reduction of sea
level initiates dissociation along the base of the gas hydrate, which, in turn, causes the release of large
volumes of gas into the sediment and creates excess pore fluid pressures and reduced slope stability.
In such settings, the overlying permafrost cap may act as a permeability barrier, which could result in
significant excess pore pressures and reduction in sediment stability. The deformed sediments lay
above a smooth BSR, suggesting that overpressures due to gas hydrate dissociation may have contributed to or caused the slides. A quantitative approach was taken to predict excess pressures at the base
of the gas hydrate zone, and the stability of the overlying sediment for a variety of sediment types.
The studies suggest that eustatic sea level falls and fluid diffusion properties dominate the
9.5 Geological Hazards
315
FIGURE 9.16
Shaded-relief map of US Atlantic margin (gray), gridded from single-beam bathymetric soundings, and new
bathymetric data (color-coded for depth) collected with a multibeam echosounder during the May 2009
cruise. (From Brink, 2009.)
dissociation process in fine-grained marine sediment. Considering the physical properties and the
pattern of sedimentation, two-dimensional modeling was carried out to ascertain the pore pressure
required for initiation of triggering of slope failure. These models give credence to the theory that
fluctuations in global climate, manifest in periodic Pleistocene eustatic sea-level regressions, likely
triggered seafloor landslides on the continental slope of the Beaufort Sea and other margins where
gas hydrate is present in the seafloor sediment (Kayen and Lee, 1991).
By combining mechanical and thermal loading of the sediment, a more accurate indication of slope
stability was obtained. The stability analysis results indicate a relatively low factor of safety for the
Beaufort sediments without the presence of permafrost and gas hydrate, owing to the relative slope
steepness compared to other submarine failures. Including the effects of the permafrost and gas hydrate
in the sediments can result in an increase of the factor of safety under static conditions. However, modeling of the temporal effects of transgression of the Beaufort Shelf (considering change in pressure and
temperature), indicates that, for a reasonable assumption of between 5 and 35% hydrate content, the
factor of safety reduces to below unity and failure occurs (Grozic and Dallimore, 2012).
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CHAPTER 9 GEOLOGICAL HAZARDS
9.5.4 CASCADIAN MARGIN
The Cascadian margin is one of the most extensively studied gas hydrate provinces confined to convergent continental margins of the world. The entire length of the Juan de Fuca oceanic plate is steeply
subducting beneath the North American plate at a rate of about 46 mm/a. This subducting plate led to
the accumulation of a 3- to 4-km-thick accretionary prism onto the American plate marked by mega
thrusts associated with intense earthquake activity with a return period of about 300−700 years. On the
northern Cascadia accretionary margin off Vancouver Island, Canada, there are numerous sedimentary
slide features near the base of the slope; notable among them are the Orca Slide and Slipstream Slide.
The presence of marine gas hydrate beneath the ridge is based on a widespread hydrate BSR, high
velocities determined by ocean bottom seismograph data, and sediment core samples and downhole
logs collected by the Integrated Ocean Drilling Program at Site U1326. The depth of the BSR at 255
(±15) m coincides closely with the estimated depth of the glide plane beneath the slide. Isolation of the
causative mechanism for slides in terms of steep dip angles, earthquakes, and localized excess pore
pressure either due to local geological configuration or due to the dissociation of gas hydrate in this
complex region is quite challenging. This suggests that the base of the slope failure is related to the
contrast between strong hydrate-cemented sediments above the BSR and underlying weak sediments
containing free gas. Strong earthquake shaking on this convergent margin likely provides the trigger for
the slide.
Gas hydrate concentration estimates are constrained by seismic, logging, and coring data and available high-resolution bathymetry helps to define slope geometries. As an important input for slide age
estimation the local sea-level history is used to calculate the boundary of the GHSZ over time. With
reduced pressure due to a sea-level low stand near the end of the Last Glacial Maximum, BSR subseafloor depths were shallower by ∼10 m compared to the current depths. Effects on the top of the gas
hydrate occurrence zone appear to be negligible. Rapid sea-level changes occurred around 9000−10,000
BP and 13,000−14,500 BP, which compares well with the estimated slide ages from 14C radiocarbon
dating, sulfate reduction rate modeling, and from the comparison of turbidity sequences used to date
the occurrence of past megathrust earthquakes. As a first step a simplified geometry and sedimentation
history has been used to calculate overpressure generation. Sediment column thermal models, based on
observed water column warming trends offshore Washington (USA), show that substantial volume of
gas hydrate along the entire Cascadia upper continental slope is vulnerable to modern climate change.
Dissociation along the Washington sector of the Cascadia margin alone has the potential to release
45–80 Tg of methane by 2100 (Hautala et al., 2014).
9.5.5 HYDRATE-ASSOCIATED RISKS FOR OIL AND GAS EXPLOITATION
The potential rewards of extracting methane from gas hydrate fields must be balanced with the risks.
Most of the oil and gas over the continental margins are located in shallow waters mostly confined to
continental shelves. Conventional oil and gas are located about 1−4 km below the seafloor. Gas hydrates
on the continental margins are found to occur >500 m below the seafloor, most likely on the continental
slope or rise. At these seafloor depths hydrates are formed in shallow subsurface layers <1 km below
the seafloor. Exploration and extraction of oil and gas from deeper water may encounter risk due to
destabilization of gas hydrates as extraction of oil and gas from deeper levels has to pass through the
hydrated layer. Any small perturbance in the temperature or pressure regime due to deep-sea activity
may dissociate gas hydrates causing unknown changes in the host rock matrix and thereby to the
9.5 Geological Hazards
317
wellheads: blow out preventers, pipe lines, installations and anchoring support, and to the platform
itself (Hovland and Gudmestad, 2001). Decomposition of gas hydrate reduces the sediment strength,
releases excessive gas and water, and increases the pore pressure at shallow depth leading to mass wasting, such as slope failures and deformation of sediment strata. Thus, as a general principle, either creation or dissociation of hydrate can cause problems, though dissociation is the more immediate concern.
To meet such eventualities associated with gas hydrate dissociation, some precautionary measures have
to be undertaken before the deep-sea explorations are launched. The formation and dissociation of gas
leads to substantial changes in the physical properties of subsurface sediments. Observance of BSRs in
seismic data has been considered to be a good inference of gas hydrate and free gas below the hydrated
layer. Seafloor collapse (pockmarks), gas venting, transparency of sediment, gas hydrate molds, and
mud volcanoes are considered major indicators of gas hydrate formation and dissociation in the region.
Any exploratory activity for oil and gas in deepwater may consider these as “a priori” for precautionary
measures to be undertaken.
The fluidization of sediment due to decomposition of gas hydrates and release of excessive gas and
water specifically in shallower depths leads to compaction of sediments in the producing zone and over
burden, destabilization of faults, sand production, and mass wasting as debris flow due to slope failure
or other causes that may potentially damage the infrastructure (Birchwood et al., 2010).
Most of the methane hydrates are found in shallow subsurface sediments. That means over the continental margins drilling rigs must be able to reach down through more than 500 m of the water column
and then through the hydrates layer, which is generally located hundreds of meters below the seafloor
before the extraction of oil or gas can begin. Drilling through the layer has to be done with caution as
any temperature rise or pressure alteration due to drilling mud may make the gas hydrate dissociate,
leading to reduction in sediment strength and thereby causing damage to drilling installations. To overcome this aspect, the entire sediment column of the hydrate layer has to be covered with casing, which
works out to be an expensive proposition.
Submarine gas hydrate deposits are found on continental slopes, where gradients in slope may vary
significantly from region to region. Convergent margin slope topography may be quite irregular when
compared to smooth slope topography of the divergent margins. Drilling in these oceanic deposits
could destabilize the seabed, causing vast swaths of sediment to slide for miles down the continental
slope. Evidence suggests that such underwater landslides have occurred in the past, with devastating
consequences. The movement of so much sediment would certainly trigger massive tsunamis similar to
those seen in the Indian Ocean tsunami of December 2004. The change in gradients across the slope
and abyssal plane make laying of the pipeline a challenging task. Mass movement due to slope failure
and debris flow could rupture the pipeline laid on the sea bottom and the problem is more serious for
buried pipelines. If gas hydrates dissociate suddenly and release expanded gas during offshore drilling,
it could disturb the marine sediments and compromise the pipelines and production equipment on the
seafloor. Even if you can situate a rig safely, methane hydrate is unstable once it is removed from the
high pressures and low temperatures of the deep-sea. Methane begins to escape even as it is being
transported to the surface. Unless there is a way to prevent this leakage of natural gas, extraction will
not be efficient. It will be a bit like hauling up well water using a pail riddled with holes. The decomposition of the hydrate may cause a pressure increase in the well-bore, gasification, and a possible
blowout. The presence of gas hydrates has hindered attempts to plug the Deepwater Horizon oil blowout in the Gulf of Mexico, and may have had some role in contributing to the anomalous gas pressure
in the well-bore that caused the blowout itself.
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CHAPTER 9 GEOLOGICAL HAZARDS
Gas hydrate production may lead to uncontrolled gas release during the drilling and damage the
well casing during and after the installation of the well. Offshore drilling that disturbs the gas hydratebearing marine sediments could fracture or disturb the bottom sediments and compromise the wellbore, pipelines, rig support, and other equipment used in oil and gas production from the seafloor.
Bily and Dick (1974) showed that hydrate decomposition during, and subsequent to, penetration of
the hydrate zone can be controlled by lowering the temperature of the circulating mud. It remains
important, however, to detect the presence of a hydrate zone since, in deeper drilling, higher mud temperatures cannot be avoided and such zones require adequate casing. Davidson et al. (1978) reported
several examples of gas kicks associated with hydrate decomposition. Chilling of the drilling mud,
close density control, and continuous monitoring of mud gases are now routine procedures in drilling
hydrate-prone areas of northern Canada.
Briaud and Chaouch (1997) propose a model of gas hydrate dissociation beneath the oil platform
due to the heat released around pipes where hot oil travels from the well to the platform. They report
that the melting process generates a large amount of gas that can endanger the stability of the foundation. Delisle et al. (1998) propose a model of gas hydrate formation due to the thermal re-equilibration
occurring after slumps. The main result suggests that the structure does not regain complete thermal
equilibrium after slumping in the course of several tens of years. Both Briaud and Chaouch (1997) and
Delisle et al. (1998) emphasize that the latent heat greatly impedes gas hydrate formation and dissociation: additional heat source and heat sink are produced as the gas hydrate forms and dissociates
respectively.
Destabilization of natural hydrates has occasionally affected the integrity of the seafloor or boreholes, led to well control problems, or contributed to shallow water flows (e.g., Dutta et al., 2010). Gas
hydrates in near-mud-line subsea sediments present significant challenges in the production of underlying hydrocarbons, impacting well-bore integrity and placement of subsea equipment. As the fluids of
an underlying reservoir flow to the mud-line, heat carried by the fluids warms near-well sediments and
dissociates hydrates which release gas that can displace and fracture near-well soil (Peters et al., 2008).
The long-time industry practice of simply avoiding areas with known gas hydrates during production activities that target deeper, conventional hydrocarbons has become increasingly impractical with
the push for more deepwater operations.
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